Stepped punch

ABSTRACT

A punch for forming a punched opening in a metal sheet having an elongated body with a first and second spaced apart ends. The first end is adapted to be held by a machine which, when activated, axially drives the body. The second end of the body includes an annular surface corresponding in shape and size to the punched opening as well as an extension which protrudes outwardly from and is surrounded by the annular surface. The extension has a length greater than the thickness of the sheet metal so that, as the punch is driven through the sheet metal, the extension forms a hole in the sheet metal smaller than the opening and thereafter the annular surface forms the opening of the desired size in the metal sheet.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to punches and, moreparticularly, to a punch for forming openings in metal sheets.

II. Description of Related Art

In the formation of metal parts, metal sheets are oftentimes stampedwith metal punches to form the desired or required openings. The metalsheet is then formed into the desired end part by stamping. During thatforming process, one or more openings may be formed within the metalsheet by metal punches.

For example, in the process of hot metal stamping, a metal sheet ofcarbon steel is formed into its desired shape by a die and then heatedto an elevated temperature at which the metal forms into the austenitephase. At that time, metal punches are used to punch openings within themetal sheet. The stamped part is then rapidly cooled or quenched whichtransforms the metal from austenite to martensite. The martensite phaseof steel is very hard and used in applications where very hard metal isrequired, e.g. crash barriers for automotive vehicles.

One problem in the hot stamping operation is that, when the heated partis punched by metal punches, a relatively large amount of flash isformed around a portion of the punched hole. When the part is quenched,that flash also hardens and must be removed. Such removal usuallyrequires extensive grinding of the part which adds to the overallmanufacturing cost of the part.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a punch for forming a punched opening ina metal sheet that overcomes the above-mentioned disadvantages of thepreviously known punches.

In brief, the punch of the present invention comprises an elongated bodyhaving a first and second spaced ends. The first end is adapted to beheld by a machine which, when activated, axially drives the body. Thataxial direction is typically normal to the surface in which the punchedhole is formed.

The second end of the body includes an annular surface corresponding inshape and size to the desired punched hole in the metal sheet. Anextension protrudes outwardly from and is surrounded by the annularsurface.

The extension has a length greater than the thickness of the metal sheetso that, as the punch is driven through the metal sheet, the extensionfirst forms a hole in the metal sheet smaller than the desired endopening in the metal sheet. As the punch is further driven into themetal sheet, the annular surface then contacts and forms the desiredopening in the metal sheet. In practice, however, by first forming asmall opening in the metal sheet, the formation of flash around thefinal punched hole is minimized. This is especially true when the punchis used in a hot metal stamping process.

BRIEF DESCRIPTION OF THE DRAWING

A better understanding of the present invention will be had uponreference to the following detailed description when read in conjunctionwith the accompanying drawing, wherein like reference characters referto like parts throughout the several views, and in which:

FIG. 1 is an elevational view of a punch according to the presentinvention;

FIG. 2 is an end view of the punch; and

FIGS. 3 and 4 are fragmentary diagrammatic views illustrating theoperation of the punch of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION

With reference to FIGS. 1 and 2, a punch 10 according to the presentinvention is shown. The punch 10 includes an elongated body 12 having afirst end 14 and a second end 16. The first end 14 of the body isadapted to be mounted in a machine 18 (illustrated onlydiagrammatically) which, when activated, axially drives the punch 10along the axis of the body 12. Furthermore, the punch 10 of the presentinvention is designed in particular for punching holes in heated sheetmetal during a hot stamping process.

The body 12 includes a punch section 20 adjacent the punch end 16. Thispunch section 20 has a cross-sectional shape equal to the shape of thedesired final hole in the metal sheet. The punch section 20 tapersoutwardly into a mounting portion 22 of the body 12. This mountingportion 22 is greater in cross-sectional area than the punch section 20and is dimensioned to fit into the machine 18.

Still referring to FIGS. 1 and 2, the punch section 20 terminates in anannular surface 24. This annular surface 24 is generally perpendicularto the direction 26 of movement of the punch 10 when driven by themachine 18.

An extension 30 of the body 12 protrudes axially outwardly from and issurrounded by the annular surface 24. This extension 30 terminates atthe free end 16 of the punch body 12. Furthermore, the cross-sectionalarea of the extension 30 is preferably at least 70% of thecross-sectional area of the punch section 20. In addition, the end 16 ofthe extension 30 lies in a plane generally perpendicular to the punchaxis 26.

A rounded chamfer 32 is preferably formed at the junction of theextension 30 and the annular surface 24. This chamfer not only relievespossible stress within the punch 10, but also prevents punch remnantsfrom becoming stuck on the punch 10.

The entire punch 10 is preferably of a one-piece construction and formedfrom hardened metal. As such, both the free end of the punch extension30 as well as the annular surface 24 of the punch section 20 are capableof forming holes in the metal sheet.

With reference now to FIGS. 3 and 4, upon activation of the machine 18,the machine 18 drives the punch 12 along its axis 26 toward the metalsheet 34. In doing so, the extension 30 first contacts the metal sheet34 and punches a relatively small hole 36 in the metal sheet 34 as shownin FIG. 3. This punching operation also produces a small punched slug 38from the metal sheet 34.

As the machine 18 continues to drive the punch 10 toward the metal sheet34 to the position shown in FIG. 4, the annular surface 24 contacts themetal sheet and forms a hole 39 of the desired size in the metal sheet34. This annular surface thus forms a slug 40 in the shape of a washerfrom the metal sheet 34. However, the chamfer 32 between the annularsurface 24 and the extension 30 prevents the slug 40 from being stuck onthe punch 10. The punch 10 is then retracted from the metal sheet 34thus completing the hole 38 in the metal sheet 34.

In operation, it has been found that by using the two-step punch to formholes 38 in the metal sheets 34, very little flashing is formed aroundthe punched hole 38. Indeed, in many cases, the flashing is so minoraround the formed hole 38 that further removal or grinding of theflashing is unnecessary.

From the foregoing, it can be seen that the present invention provides apunch for forming openings in metal sheets and which is particularlyuseful during a hot stamp metal process. Having described my invention,however, many modifications thereto will become apparent to thoseskilled in the art to which it pertains without deviation from thespirit of the invention as defined by the scope of the appended claims.

I claim:
 1. A punch for forming a punched opening in a metal sheetcomprising: an elongated body having a first and a second spaced ends,said first end adapted to be held by a machine which, when activated,axially drives the body, said body having a punch section axially spacedfrom said second end of said body, said punch section terminating in anannular surface corresponding in shape and size to the punched opening,and an extension which extends axially outwardly from and is surroundedby said annular surface, said extension terminating at said second endof said body, wherein said extension has a length greater than athickness of the metal sheet so that, as the punch is driven through themetal sheet, said extension forms a hole in the metal sheet smaller thanthe opening and thereafter said annular surface forms the opening in themetal sheet.
 2. The punch as defined in claim 1 wherein said punch isformed from hard metal.
 3. The punch as defined in claim 1 andcomprising a chamfer extending around a junction between said annularsurface and said extension.
 4. The punch as defined in claim 3 whereinsaid chamfer is formed on a radius.
 5. The punch as defined in claim 1wherein said body is a one piece construction.
 6. The punch as definedin claim 1 wherein a free end of said extension lies in a planeperpendicular to said predetermined direction.
 7. The punch as definedin claim 1 wherein said annular surface lies in a plane perpendicular tosaid predetermined direction.
 8. The punch as defined in claim 1 whereinsaid extension is circular in cross-sectional shape.
 9. The punch asdefined in claim 1 wherein both said annular surface and said extensionare circular in cross-sectional shape and wherein said extension has adiameter greater than sixty percent of the diameter of said annularsurface.